ExploderWizard
Hero
The concept was that some weapons were better/worse versus different types of armour. Chainmail was thought to be better than plate against arrows, for example.
It wasn't. The long bow had a +1AC adjustment vs. chain and a 0 AC adjustment vs. plate.
The trouble is that this is not how the table worked! You cross-referenced the weapon you were using, not against the type of armour your target was wearing, but against the target's base AC! Different armour types could have the same base AC, and IIRC using a shield added to that base AC.
Check the note on weapon types "to hit" adjustment in the AD&D DMG page 28. There it clarifies that these adjustments are for specific types of armor rather than armor class.
Although it was a cool idea, the actual table wasn't fit for purpose.
It was far too much of a pain in the rear to be very useful in play for certain.